Why is old glass wavy?

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Old glass has wavy imperfections because of the way it was made, not because it continues to flow. That's a commonly repeated myth. Very old glass was made by spinning a glass crown disk from a central rod, flattening it, and cutting rectangular pieces out of it. Later, in the early 1900s, glass was made in tall cylinders which were reheated and flattened. After the mid 20th century it has been made by floating molten glass on molten tin, which results in perfectly smooth glass. Whichever way it's made, it doesn't change shape appreciably after being installed in a window.

- from James Gurney's blog [1]